As a water-based ink for ink-jet recording, The pigmented ink has been developed. The pigmented ink is expected to have excellent water resistance and light fastness, but has problems such as change over time due to aggregation or sedimentation of the pigment, and clogging of a nozzle of print head. To solve these problems, a method of dispersing a pigment in an aqueous medium using a resin as a dispersant is employed. As this method, there has been proposed a method of dispersing a resin which has an acid value in water in the presence of a basic component to obtain an aqueous solution, adding a pigment, sufficiently stirring the mixture, and dispersing the pigment using a paint shaker or a sand mill (see Patent Document 1). However, since this method does not include the process of producing a colored kneaded mixture by a kneading prior to dispersing process, long dispersing process time is required, causing a problem such as low production efficiency. The resin used as a dispersant is not selected enough in this method. Since the water-based pigment dispersion thus obtained is insufficient in dispersion stability, it is difficult to stably disperse the pigment for a long period of time by using only the resin as the dispersant. Furthermore, it is impossible to realize good ejection properties of the ink, and good image quality and durability of printed images. Thus, a water-based pigmented ink composition for ink-jet recording having excellent long-term storage stability has never been provided.
As for resin used as dispersant, a styrene-based resin has been studied. For example, there is known an example of a water-based pigment dispersion for ink-jet ink in which a mixture of a humectant, sodium hydroxide and carbon black is dispersed by using styrene-acrylic resin (styrene/acrylic acid/methacrylic acid=77/10/30, molecular mass: 7200 and 8300) as the dispersant with a paint shaker (see Patent Document 2). According to the method of producing a water-based pigment dispersion described in the above publication, it is made possible to produce a water-based ink for ink-jet recording having excellent dispersion stability in which carbon black has a fine particle size, and the water-based ink is excellent in ink ejection properties. However, since the kneading process for producing a colored kneaded mixture is not included and an effective auxiliary dispersant is not used, the water-based ink was insufficient in long-term dispersion stability when used for a thermal jet type ink-jet printer. When this method is applied to an azo pigment or quinacridone pigment, which is considered to have poor dispersibility, good dispersibility is not attained as compared with carbon black.
Trials adding various pigment derivatives as auxiliary dispersants other than using the resin as the dispersant, were made so as to stably maintain a dispersion state of the pigment for a long period.
For example, there is proposed a method of utilizing electrostatic repulsion of a sulfonic acid group-containing pigment derivative adsorbed onto the surface of a pigment using a pigment and a sulfonic acid group-containing pigment derivative (see Patent Document 3). According to this method, although dispersion stability of a colorant is improved, printed matter obtained has high affinity with water because of the presence of a polar functional group on the surface of the colorant. Furthermore, according to this method, dispersion stability is improved only by the addition of the pigment derivative, and thus there may arise a problem of poor water resistance.
To attain excellent dispersibility and dispersion stability of a water-based pigment dispersion or recording ink, which contains a quinacridone-based pigment as the colorant, there are proposed an encapsulated pigment dispersion wherein a quinacridone-based pigment and a dimethylaminomethylated quinacridone compound are used in combination (see Patent Document 4), and a water-based pigment dispersion containing a quinacridone-based pigment, a phthalimidomethylated quinacridone-based compound, a quinacridonesulfonic acid-based compound, and a anionic group-containing organic polymeric compound having a glass transition point of −20 to 60° C. (see Patent Document 5). However, the magenta water-based pigment dispersion described in these publications has never been studied regarding selection of the resin used as dispersant and the kneading method suited for the resin to be used, and thus it was not sufficient in dispersion level and dispersion stability.
It was difficult to produce a water-based pigment dispersion with good dispersibility, containing a quinacridone pigment having poor dispersibility, only by using the dispersant or auxiliary dispersant.
To improve dispersibility of the water-based pigment dispersion in the production process, a mixture of a resin and a pigment, or a mixture of an aqueous resin solution containing a resin, water, a water-soluble organic solvent, and a pigment is previously milled by rollers prior to the dispersing step. In the method of milling using twin rollers, there is employed a method of kneading the mixture to obtain solid chips containing a pigment, mainly adding water and a water-soluble organic solvent to the solid chips, and dispersing them using a high-speed mixer or homogenizer to obtain a water-based pigment dispersion (see, for example, Patent Documents 6 and 7).
To easily produce a resin solution, an organic amine compound is added during kneading and a specific pigment derivative is also used (see, for example, Patent Document 8).
A water-based pigment dispersion is produced by kneading a styrene-acrylic resin having a weight-average molecular weight of 50000 using twin rollers (see, for example, Patent Document 9).
By using the above method, the pigment is finely ground between rollers under shear, and coating with the resin as the dispersant proceeds on the surface of the finely ground pigment. To optimize kneading conditions of the mixture of the resin and the pigment, kneading is carried out after adding a solvent thereby to dissolve the resin, or melting the entire mixture. Therefore, in the case of preparing a water-based pigment dispersion from the kneaded mixture, removal of the solvent and crushing of the melted kneaded mixture are required and the dispersing time is prolonged, and thus a water-based pigment dispersion having good dispersion stability is not always obtained. For example, kneading using twin rollers is conducted in an open system and water and the water-soluble organic solvent are vaporized during kneading and, finally, solid chips having high solid content are obtained. In the dispersing step, crushing and dispersing of the pigment in the form of solid chips must be conducted after adding water and the water-soluble organic solvent. Since the pigment coated with the resin as the dispersant is crushed, the coating of the surface of the pigment may become insufficient.
Only by passing through the kneading step, the pigment in the water-based pigment dispersion is finely ground and initial dispersibility is improved. However, the water-based pigment dispersion is insufficient in long-term dispersion stability under a high-temperature environment on the assumption that it is used for a thermal jet type ink-jet printer.
To obtain a satisfactory aqueous dispersion of a quinacridone-based pigment having poor dispersibility, it is important to use a method capable of effectively realizing coating of the surface of the pigment with the resin and adsorption of the auxiliary dispersant onto the surface of the pigment by selecting the resin as the dispersant and an auxiliary dispersant.
However, there has never been found a method of producing an ink composition for ink-jet recording containing a quinacridone-based pigment, capable of simultaneously realizing dispersibility, long-term dispersion stability, and ink ejection properties during image printing at a high level. Thus, it has been strongly required to develop an ink composition for ink-jet recording which is used to satisfactorily perform thermal jet type ink-jet recording, and a method of producing a water-based pigment dispersion for ink-jet ink used to produce the above ink composition.    Patent Document 1: Japanese Patent Application, First Publication No. Hei 10-60332    Patent Document 2: Japanese Patent Application, First Publication No. 2002-256201    Patent Document 3: Japanese Patent Application, First Publication No. 2002-241638    Patent Document 4: Japanese Patent Application, First Publication No. Hei 9-151342    Patent Document 5: Japanese Patent Application, First Publication No. 2000-191974    Patent Document 6: Japanese Patent Application, First Publication No. Hei 6-157954    Patent Document 7: Japanese Patent Application, First Publication No. 2000-80299    Patent Document 8: Japanese Patent Application, First Publication No. 2001-81390    Patent Document 9: Japanese Patent Application, First Publication No. Hei 10-88042
An object of the present invention is to provide a red or magenta water-based ink composition for ink-jet recording, which is excellent in dispersibility and dispersion stability and is less likely to cause an increase in particle size even when allowed to stand at high temperature for a long period, and also which is less likely to cause an increase in viscosity and is excellent in printing stability, a water-based pigment dispersion for ink-jet ink used to produce the same, and a method of producing the same.
Another object of the present invention is to provide a water-based ink composition for an ink-jet recording suited for a thermal ink-jet recording system.